This invention relates to improved protective silicone resin coating compositions. More particularly, it relates to a silicone resin coating composition containing a silylated ultraviolet radiation screening agent (UV screen) which will adhere readily to unprimed plastic substrates to form a tough, abrasion-resistant coating thereon.
Recently, the substitution of glass with transparent materials which do not shatter, or are more resistant to shattering than glass, has become widespread. For example, transparent glazing made from synthetic organic polymers is now utilized in public transportation vehicles such as trains, buses, taxis and airplanes. Lenses for eye glasses and other optical instruments, as well as glazing for large buildings, also employ shatter-resistant, transparent plastics. The lighter weight of these plastics in comparison to glass is a further advantage, especially in the transportation industry where the weight of the vehicle is a major factor in fuel economy.
While transparent plastics provide a major advantage of being more resistant to shattering than glass, a serious drawback lies in the ease with which these plastics mar and scratch due to everyday contact with abrasives, dust and cleaning equipment, and ordinary weathering. Continuous scratching and marring results in imparied visibility and poor aesthetics, and often requires replacement of the glazing or lens or the like.
One of the most promising and widely used transparent plastics for glazing is polycarbonate, such as that known as LEXAN.RTM., sold by General Electric Company. It is a tough material, having high impact strength, high heat deflection temperature and good dimensional stability. It is also self-extinguishing, and easily fabricated. Acrylics, such as polymethylmethacrylates, are also widely used glazing and lens materials.
Attempts have been made to improve the abrasion resistance of these transparent plastics. For example, scratch-resistant coatings formed from mixtures of silica, such as colloidal silica or silica gel, and hydrolyzable silanes in a hydrolysis medium, such as alcohol and water, are known. Misch, et al., U.S. Pat. No. 3,708,225; Clark, U.S. Pat. No. 3,986,977; 3,976,497 and 4,027,073; Armbruster, et al., U.S. Pat. No. 4,159,206; and Ubersax, U.S. Pat. No. 4,177,315, for example, describe such compositions. Improved such compositions are also described in commonly assigned copending U.S. application Ser. No. 964,910, filed Nov. 30, 1978, and Frye, U.S. Pat. No. 4,277,287.
It has been discovered that such polysilicic acid coatings, especially if acidic, fail to adhere to certain plastic substrates, such as polycarbonate, and, even if prepared on the basic side of neutrality, they may adhere initially but peel after brief light aging. In copending application Ser. No. 91,716, filed Nov. 6, 1979, now U.S. Pat. No. 4,299,746, issued Nov. 10, 1981, the addition of an ultraviolet (UV) light absorbing agent, such as 2,4-dihydroxybenzophenone, is suggested, but in some cases this may have a plasticizing effect, and polycarbonate seems to have a tendency to reject the coating on severe exposure. Another approach is to use a primer coat which adheres to both the silicone resin coating and the polycarbonate substrate, and which also serves as a binder for high levels of UV absorbers. The silicone resin coating is put on as a second coat over the primer coat. See, for example, Humphrey, Jr., U.S. Pat. No. 4,188,451, and Frye, U.S. Pat. No. 4,277,287.
To avoid the need for a primer coat, commonly assigned copending U.S. application Ser. No. 34,164, filed Apr. 27, 1979 now abandoned, suggests replacing the usual solvents, e.g., isobutanol, with a more aggressive solvent, e.g., an ester, a ketone, a nitroparaffin, or the like. However, these are expensive and generally might require process modifications.
Other approaches to the elimination of primers from the application of silicone coatings to plastic are described in commonly assigned copending U.S. application Ser. Nos. 326,197, filed Dec. 1, 1981, and 154,624, filed May 30, 1980, now U.S. Pat. No. 4,373,061, issued Feb. 8, 1983. The former describes shock curing at elevated temperatures of a silicone resin coating to yield an adherent coating without the use of primers. The latter application discloses that incorporation of a silylated UV screening compound into the silicone resin admits to primerless adhesion to plastic.
There are, however, drawbacks to these approaches: High temperature curing is performed in a temperature range (about 140.degree. C. to 150.degree. C.) which may distort or warp a plastic substrate, may cause total adhesion failure instead of promoting it, or may volatilize certain beneficial additives (such as those described in the above-mentioned U.S. application Ser. No. 91,716). The use in silicone resin coatings of silylated UV screens, such as 4-(3-triethoxysilylpropoxy)-2-hydroxybenzophenone (SHBP) and the like, requires a lengthy aging period, delaying the time the coating composition is usable for a week or longer.
The above-mentioned patents and applications are incorporated herein by reference.